An image forming apparatus determines a time length of a non-image-forming period in which image formation is not performed, the non-image-forming period being from when image formation on one recording sheet ends to when image formation on the next recording sheet starts, and based on the determined time length, decides the number of times of executing measurement (BD interval measurement) of a generation timing difference between detection signals corresponding to light beams emitted from two light emitting elements. The image forming apparatus executes the decided number of times of BD interval measurement and calculates an average value of the resultant measurement values.